Perform regression on combined shallow crustal and subduction zone records, on just the shallow crustal
records using rhypo and using repi and on just subduction zone records.

Note that distribution w.r.t. distance improves in the near field when epicentral distance is used but only
possible to use repi for shallow crustal earthquakes because for subduction zone earthquakes hypocentral
distance is much greater than epicentral distance so should use rhypo instead.

For 4 ≤ M ≤ 6 distribution w.r.t. epicentral distance is quite good but for M > 6 no records from
de< 40km.

Use a two step procedure. Firstly use entire set and both horizontal components and compute two soil terms
(one for hard and one for soft soil). In second step use soil terms to correct motions for rock conditions
and then repeat regression.

Use Bayesian analysis (Ordaz et al., 1994) so that derived coefficients comply with physics of wave
propagation because include a priori information on the coefficients to avoid physically unrealistic values.
Choose initial values of coefficients based on theory and previous results

Cannot find coefficient in r by regression so adopt 6km from previous study.